[Issue 19101] Miscompilation on extern(C++) overloads with D types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19101

--- Comment #3 from kinke at gmx.net ---
(In reply to johanengelen from comment #2)
> Currently we mangle extern(D) types as if they are extern(C++) types for 
> extern(C++) templated functions.

Ah now I get it, I overlooked that the 2 structs are extern(D). [The function
doesn't need to be templated.] Not sure if a dot can be used in typenames for
C++ mangling. Also not sure how much code that would break (structs needing
`extern(C++)` for C++ interop, whereas that's optional now).

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